This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Recent developments of the Third Generation Partnership Project Long Term Evolution, 3GPP LTE, facilitate accessing local IP based services at home, office, public hot spot or even in outdoor environments. One of the important application cases for the local IP access and local connectivity involves direct communications between devices in close proximity to each other, typically less than a few 10s of meters, and sometimes up to a few hundred meters.
This kind of direct communications, which may also be referred to as Device to Device D2D communications, sometimes enables a number of potential gains over the traditional cellular technique, since in the D2D communications, communicating devices are much closer to each other than in the cellular communications, in which the devices have to communicate via a cellular access point AP or a base station. Therefore, a network structure covering both cellular and D2D communications as illustrated in FIG. 1 is becoming more attractive.
In 3GPP, it has been concluded that for an intra-user equipment UE case where the UE conducts both cellular communications and D2D communications, the cellular uplink UL transmission from the UE and D2D activity, i.e. transmission and reception will be conducted in a Time-Division Multiplexing TDM way so as to avoid collisions. As for an inter-UE case where the cellular communications and D2D communications are conducted between different UEs, although there is no clear conclusion yet, the TDM is also a promising scheme to deal with cellular/D2D collisions by considering the conclusion in 3GPP that idle D2D transmissions can use the cellular DL timing reference, which is different from the cellular UL transmission timing and the different transmission timing of the cellular and D2D communications being a cause for severe inter-frequency interference due to unorthogonality.
In order to effectively divide resources between the cellular and D2D communications in a mixed 3GPP LTE cellular and D2D network, two problems shall be considered. The first problem is how to enable D2D transmissions to avoid cellular UL control channels, including Physical Uplink Control Channel PUCCH carrying uplink control information, e.g. ACK/NACK feedback, Scheduling Request SR, Channel State Information/Pre-coding Indicator/Rank Indicator CSI/PMI/RI, Physical Random Access Channel PRACH and Sounding Reference Signal SRS. The second problem is how to enable D2D transmissions to avoid cellular UL data channels, including both Physical Uplink Shared Channel PUSCH new transmissions and Hybrid Automatic Repeat Request HARQ re-transmissions.
Among the above mentioned cellular UL transmissions, static PUCCH transmissions, e.g. SR and CSI/PMI/RI, PRACH and SRS have a fixed periodicity and can be configured by the network. Dynamic PUCCH, e.g. ACK/NACK feedback, and PUSCH new-transmissions can be dynamically controlled on a subframe basis by the network. However, for PUSCH HARQ re-transmissions, there is a strict timing relationship between respective transmission occasions, since synchronous HARQ processes are used by the cellular UL.
Therefore, it is difficult to allocate resources for the D2D communications so that it can avoid all possible cellular UL transmissions, especially in TDD Uplink-Downlink configurations (which will be abbreviated as UL-DL configurations hereafter) 0 and 6 as specified in table 4.2-2 of 3GPP TS 36.211, which are more beneficial to D2D communications since there are more UL subframes than DL subframes.
The first problem as mentioned above is touched by the recent contribution R1-135093 from CATT, which mainly focuses on how to avoid cellular PUCCH ACK/NACK feedback into the D2D resource pool. However, it does not address the above mentioned second problem, e.g. PUSCH re-transmission problem.
The second problem as mentioned above is partly addressed in a PCT patent application PCT/CN2012/086487 for a mixed cellular and D2D system, and in another PCT patent application PCT/CN2013/071191 for a dual connectivity scenario, wherein it is proposed to divide resources in a HARQ-process specific way, i.e. on a HARQ-process basis, for example process I for sub-system A, e.g. cellular, or connectivity 1 while process II for sub-system B, e.g. D2D, or connectivity 2.
As for the PUSCH re-transmission problem, the proposed solutions of PCT/CN2012/086487 and PCT/CN2013/071191 do work well in a Frequency Division Duplex FDD system, and a TDD system with UL-DL configurations 1, 2, 3 and 4 as specified in table 4.2-2 of 3GPP TS 36.211, wherein UL-DL configuration 5 does not work since there is a single UL HARQ process, and thus cannot be further divided.
However, for UL-DL configurations 0 and 6, which have more UL subframes and thus more beneficial to D2D communications, the proposals of PCT/CN2012/086487 and PCT/CN2013/071191 do not work well since the HARQ processes cannot be divided in a ‘Transmission Time Interval TTI-specific’ way. For example, a HARQ process in TTI (which can be interchangeably used with ‘subframe’ in LTE) x of a frame would run into TTI x+1 in the next frame, and finally iterates all UL subframes. The ‘TTI-specific’ division is necessary for solving the first problem, since the UL control channel other than ACK/NACK feedback are all pre-allocated in a ‘TTI-specific’ way, i.e. pre-allocated with a fixed time offset and periodicity.
In order to solve the above mentioned problem, a traditional ‘ACK PHICH (Physical Hybrid-ARQ Indicator Channel) plus non toggled NDI (New Data Indicator)’ solution can be used to prevent the PUSCH re-transmissions from occurring in a colliding subframe temporarily. For example, it is assumed that a terminal device is scheduled for an initial transmission in subframe n and if the transmission that is not correctly received, then a retransmission will be conducted in subframe n+8. With non-adaptive hybrid ARQ, the retransmission occupies the same part of the uplink spectrum as the initial transmission. Hence, in this example, the spectrum is fragmented, which limits the bandwidth available for another terminal device. In subframe n+16, an instance of an adaptive retransmission is found, e.g., via ACK in PHICH, and then the terminal device would pause the re-transmission in the current subframe but not flash the data in the buffer, while using non-toggled NDI in the next re-transmission opportunity, but the terminal device would still send the old data to the network. To make a room for another terminal device to be granted a large part of the uplink spectrum, the retransmission is moved in the frequency domain. But this scheme costs extra Downlink Control Information DCI signaling for all terminal devices in the colliding HARQ process.